--- a/client.mk	2020-12-22 05:23:56.000000000 +0300
+++ a/client.mk-OK	2020-12-24 19:54:52.834091571 +0300
@@ -281,17 +281,25 @@
 configure-preqs = \
   $(OBJDIR)/CLOBBER \
   configure-files \
   $(call mkdir_deps,$(OBJDIR)) \
   save-mozconfig \
   $(OBJDIR)/.mozconfig.json \
   $(NULL)
 
-CREATE_MOZCONFIG_JSON = $(shell $(TOPSRCDIR)/mach environment --format=json -o $(OBJDIR)/.mozconfig.json)
+# Do not try to re-create .mozconfig.json when running
+# directly under `make -f client.mk' 
+ifndef MACH
+ifneq (,$(wildcard $(OBJDIR)/.mozconfig.json))
+CREATE_MOZCONFIG_JSON =
+endif
+endif
+
+CREATE_MOZCONFIG_JSON ?= $(shell $(TOPSRCDIR)/mach environment --format=json -o $(OBJDIR)/.mozconfig.json)
 # Force CREATE_MOZCONFIG_JSON above to be resolved, without side effects in
 # case the result is non empty, and allowing an override on the make command
 # line not running the command (using := $(shell) still runs the shell command).
 ifneq (,$(CREATE_MOZCONFIG_JSON))
 endif
 
 $(OBJDIR)/.mozconfig.json: $(call mkdir_deps,$(OBJDIR)) ;